Microlithographic projection exposure apparatuses serve to produce microstructured components using a photolithographic method. In so doing, a structure-bearing mask, the so-called reticle, is imaged onto a light-sensitive layer with the aid of a projection system. A shorter wavelength of the employed imaging light allows smaller structures to be imaged onto the mask with the aid of the projection system. In so-called EUV lithography apparatuses, use is made of imaging light with a used wavelength in the extreme ultraviolet (EUV) range between approximately 5 nm and approximately 20 nm. In the case of such a used wavelength in the range between 5 nm and 20 nm, use is only made of reflective optical elements (EUV mirrors) for imaging purposes and the structure-bearing mask is also a reflecting optical element, onto which a structured absorbing layer or coating has been applied in order to generate the desired structures when imaging on the light-sensitive layer.
An EUV lithography apparatus comprises a light source for generating EUV radiation for illuminating the reticle or the mask. In addition to the desired used wavelength in the EUV wavelength range (for example at approximately 13.5 nm), currently known light sources for generating EUV radiation also emit radiation at many other wavelengths. While EUV radiation is already significantly attenuated by the reflection at a plurality of EUV mirrors placed in the beam path in succession, radiation in the DUV, UV, VIS and IR wavelength ranges may be reflected, in part without impediment, at all EUV mirrors and at the mask and may reach the light-sensitive layer or the wafer. The latter is problematic, in particular for the DUV proportion of the radiation emitted by the EUV light source, since the light-sensitive layer, i.e. the photoresist (also referred to as resist), which is applied onto the wafer, is exposed not only by the EUV radiation proportion but also by the DUV radiation proportion, with the exposure by the latter proportions being very blurred and not defined, i.e. the DUV radiation proportion does not generate precise imaging or does not generate a precise image of the structures on the mask on the wafer.
Solutions for removing DUV radiation before it may reach the light-sensitive layer are known, but, in practice, these solutions are almost always connected with a significant light loss in the EUV wavelength range. It is therefore desirable to be able to use EUV lithography apparatuses without DUV suppression or with only a weak DUV suppression.
It is essential for the use of such EUV lithography apparatuses to precisely know what proportion of the contrast on the light-sensitive layer or on the wafer is caused by the DUV radiation proportion in the radiation emitted by the EUV light source. This contrast proportion may not be too large because, otherwise, imaging of the structures formed on the mask is carried out with an insufficient contrast. The DUV radiation proportion depends strongly on the respective settings of the EUV light source or of the illumination system; the contrast at the respective wavelength depends, inter alia, on the employed resist.
U.S. Pat. No. 6,593,037 B1 discloses a reflective mask (reticle) for EUV lithography, said mask being embodied to reduce reflections at an absorbing coating at wavelengths shorter than in the DUV wavelength range. The reflective mask is embodied to generate additional reflections which have a desired phase difference in respect of the reflections at the absorbing coating such that the additional reflections reduce or eliminate the reflections at the absorbing coating by way of destructive interference. The absorbing coating may have two or more layers, the layer thicknesses and layer number of which being selected in such a way that reflections at a specific EUV wavelength are minimized. The absorbing coating is typically applied to a buffer layer which is arranged above a multilayer coating for reflecting EUV radiation.